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This invention relates to dielectric resonator antennas with steerable receive and transmit beams and more particularly to an antenna having several separate feeds such that several separate beams can be created simultaneously and combined as desired.
Since the first systematic study of dielectric resonator antennas (DRAs) in 1983 (LONG, S. A., McALLISTER, M. W., and SHEN, L. C.: xe2x80x98The resonant cylindrical dielectric cavity antennaxe2x80x99, IEEE Trans. Antennas Propagat., AP-31, 1983, pp 406-412), interest has grown in their radiation patterns because of their high radiation efficiency, good match to most commonly used transmission lines and their small physical size (MONGIA, R. K. and BHARTIA, P.: xe2x80x98Dielectric resonator antennasxe2x80x94A review and general design relations for resonant frequency and bandwidthxe2x80x99, Int. J. Microwave and Millimetre Wave Computer-Aided Engineering, 1994, 4, (3), pp 230-247). Most configurations reported have used a slab of dielectric material mounted on a ground plane excited by either an aperture feed in the ground plane or by a probe inserted into the dielectric material. A few publications have reported on experiments using two probes fed simultaneously in a circular dielectric slab. These probes were installed on radials at 90xc2x0 to each other and fed in anti-phase so as to create circular polarisation (MONGIA, R. K., ITTIPIBOON, A., CUHACI, M. and ROSCOE D.: xe2x80x98Circular polarised dielectric resonator antennaxe2x80x99, Electron. Lett., 1994, 30, (17), pp 1361-1362; and DROSSOS, G., WU, Z. and DAVIS, L. E.: xe2x80x98Circular polarised cylindrical dielectric resonator antennaxe2x80x99, Electron. Lett., 1996, 32, (4), pp 281-283.3, 4) and one publication included the concept of switching probes on and off (DROSSOS, G., WU, Z. and DAVIS, L. E.: xe2x80x98Switchable cylindrical dielectric resonator antennaxe2x80x99, Electron. Lett., 1996, 32, (10), pp 862-864). 
All references mentioned herein are incorporated herein by reference:
The present invention seeks to provide a DRA having several probes or aperture feeds connected in such a way that the antenna pattern can be steered, and also the use of two probes driven simultaneously in-phase and 180xc2x0 out of phase in order to generate monopulse sum and difference patterns.
One method of electronically steering an antenna pattern is to have a number of existing beams and to switch between them, or to combine them so as to achieve the desired beam direction. A circular DRA may be fed by a single probe or aperture placed in or under the dielectric and tuned to excite a particular resonant mode. In preferred embodiments, the fundamental HEM11xcex4 mode is used, but there are many other resonant modes which produce beams that can be steered equally well using the apparatus of embodiments of the present invention. The preferred HEM11xcex4 mode is a hybrid electromagnetic resonance mode radiating like a horizontal magnetic dipole and giving rise to vertically polarised cosine or figure-of-eight shaped radiation pattern (LONG, S. A., McALLISTER, M. W., and SHEN, L. C.: xe2x80x98The resonant cylindrical dielectric cavity antennaxe2x80x99, IEEE Trans. Antennas Propagat., AP-31, 1983, pp 406-412). Modelling by the present Inventors of cylindrical DRAs by FDTD (Finite Difference Time Domain) and practical experimentation has shown that if several such probes are inserted into the dielectric and one is driven whilst all the others are open-circuit then the beam direction can be moved by switching different probes in and out. Furthermore, by combining feeds in different ways, sum and difference patterns can be produced which allow continuous beam-steering and direction finding by amplitude-comparison, monopulse or similar techniques.
Many of these results are described in the paper KINGSLEY, S. P. and O""KEEFE, S. G., xe2x80x9cBeam steering and monopulse processing of probe-fed dielectric resonator antennasxe2x80x9d, IEE proceedingsxe2x80x94Radar Sonar and Navigation, 146, 3, 121-125, 1999, the disclosure of which is incorporated into the present application by reference.
It has been noted by the present inventors that the results described in the above reference apply equally to DRAs operating at any of a wide range of frequencies, for example from 1 MHz to 100,000 MHz and even higher for optical DRAs. The higher the frequency in question, the smaller the size of the DRA, but the general beam patterns achieved by the probe/aperture geometries described hereinafter remain generally the same throughout any given frequency range. Operation at frequencies substantially below 1 MHz is possible too, using dielectric materials with a high dielectric constant.
According to a first aspect of the present invention, there is provided a dielectric resonator antenna including a grounded substrate, a dielectric resonator disposed on the grounded substrate and a plurality of feeds for transferring energy into and from different regions of the dielectric resonator, the feeds being activatable individually or in combination so as to produce at least one incrementally or continuously steerable beam which may be steered through a predetermined angle.
According to a second aspect of the present invention, there is provided a dielectric resonator antenna system including a grounded substrate, a dielectric resonator disposed on the grounded substrate, a plurality of feeds for transferring energy into and from different regions of the dielectric resonator, and electronic circuitry adapted to activate the feeds individually or in combination so as to produce at least one incrementally or continuously steerable beam which may be steered through a predetermined angle.
Advantageously, the antenna and antenna system of the present invention are adapted to produce at least one incrementally or continuously steerable beam which may be steered through a complete 360 degree circle.
Advantageously, there is additionally or alternatively provided electronic circuitry to combine the feeds to form sum and difference patterns to permit radio direction finding capability of up to 360 degrees.
The electronic circuitry may additionally or alternatively be adapted to combine the feeds to form amplitude or phase comparison radio direction finding capability of up to 360 degrees.
Preferably, radio direction finding capability is a complete 360 degree circle.
The feeds may take the form of conductive probes which are contained within or placed against the dielectric resonator or may comprise aperture feeds provided in the grounded substrate. Aperture feeds are discontinuities (generally rectangular in shape) in the grounded substrate underneath the dielectric material and are generally excited by passing a microstrip transmission line beneath them. The microstrip transmission line is usually printed on the underside of the substrate. Where the feeds take the form of probes, these may be generally elongate in form. Examples of useful probes include thin cylindrical wires which are generally parallel to a longitudinal axis of the dielectric resonator. Other probe shapes that might be used (and have been tested) include fat cylinders, non-circular cross sections, thin generally vertical plates and even thin generally vertical wires with conducting xe2x80x98hatsxe2x80x99 on top (like toadstools). Probes may also comprise metallized strips placed within or against the dielectric. In general any conducting element within or against the dielectric resonator will excite resonance if positioned sized and fed correctly. The different probe shapes give rise to different bandwidths of resonance and may be disposed in various positions and orientations (at different distances along a radius from the center and at different angles from the center, as viewed from above) within or against the dielectric resonator so as to suit particular circumstances. Furthermore, there may be provided probes within or against the dielectric resonator which are not connected to the electronic circuitry but instead take a passive role in influencing the transmit/receive characteristics of the dynamic resonator antenna, for example by way of induction.
In one embodiment of the present invention, the dielectric resonator may be divided into segments by conducting walls provided therein, as described, for example, in TAM, M. T. K. AND MURCH, R. D., xe2x80x98Compact circular sector and annular sector dielectric resonator antennasxe2x80x99, IEEE Trans. Antennas Propagat., AP-47, 1999, pp 837-842.
Where the dielectric resonator is of generally cylindrical form having a substantially vertical longitudinal axis, for example, the conducting walls are advantageously disposed in a substantially vertical orientation.
The dielectric resonator need not be cylindrical and may have cross-sections other than circular. For example, the resonator may have an oval cross-section or may be annular with a hollow center.
In a further embodiment of the present invention, there may additionally be provided an internal or external monopole antenna which is combined with the dielectric resonator antenna so as to cancel out backlobe fields or to resolve any front/back ambiguity which may occur with a dielectric resonator antenna having a cosine or xe2x80x98figure of eightxe2x80x99 radiation pattern. The monopole antenna may be centrally-disposed within the dielectric resonator or may be mounted thereupon or therebelow and is activatable by the electronic circuitry. In embodiments including an annular resonator with a hollow center, the monopole could be located within the hollow center. A xe2x80x9cvirtualxe2x80x9d monopole may also be formed by the electrical or algorithmic combination of any probes or apertures, preferably a symmetrical set of probes or apertures.
The dielectric resonator antenna and antenna system of the present invention may be operated with a plurality of transmitters or receivers, these terms here being used to denote respectively a device acting as source of electronic signals for transmission by way of the antenna or a device acting to receive and process electronic signals communicated to the antenna by way of electromagnetic radiation. The number of transmitters and/or receivers may or may not be equal to the number of feeds to the dielectric resonator. For example, a separate transmitter and/or receiver may be connected to each feed (i.e. one per feed), or a single transmitter and/or receiver to a single feed (i.e. a single transmitter and/or receiver is switched between feeds). In a further example, a single transmitter and/or receiver may be (simultaneously) connected to a plurality of feedsxe2x80x94by continuously varying the feed power between the feeds the beam and/or directional sensitivity of the antenna may be continuously steered. A single transmitter and/or receiver may alternatively be connected to several non-adjacent feeds to the dielectric resonator, thereby enabling a significant increase in bandwidth to be attained as compared with a single feed (this is advantageous because DRAs generally have narrow bandwidths). In yet another example, a single transmitter and/or receiver may be connected to several adjacent or non-adjacent feeds in order to produce an increase in the generated or detected radiation pattern, or to allow the antenna to radiate or receive in several directions simultaneously.
The dielectric resonator may be formed of any suitable dielectric material, or a combination of different dielectric materials, having an overall positive dielectric constant k; in preferred embodiments, k is at least 10 and may be at least 50 or even at least 100; k may even be very large e.g. greater than 1000, although available dielectric materials tend to limit such use to low frequencies. The dielectric material may include materials in liquid, solid or gas states, or any intermediate state. The dielectric material could be of lower dielectric constant than a surrounding material in which it is embedded.
By seeking to provide a dielectric resonator antenna capable of generating multiple beams which can be selected separately or formed simultaneously and combined in different ways at will, embodiments of the present invention may provide the following advantages:
i) By choosing to drive different probes or apertures, the antenna can be made to transmit or receive in one of a number of preselected directions (in azimuth, for example). By sequentially switching round the probes or apertures the beam pattern can be made to rotate incrementally in angle. Such beam-steering has obvious applications for radio communications, radar and navigation systems.
ii) By combining two or more beams together, i.e. exciting two or more probes or apertures simultaneously, beams can be formed in any arbitrary azimuth direction, thus giving more precise control over the beamforming process.
iii) By electronically continuously varying the power division/combination between two beams, the resultant combination beam direction can be steered continuously.
iv) On receive-only, the direction of arrival of an incoming radio signal can be found by comparing the amplitude of the signal on two or more beams, or by carrying out monopulse processing of the signal received on two beams. xe2x80x98Monopulse processingxe2x80x99 refers to the process of forming sum and difference patterns from two beams so as to determine the direction of arrival of a signal from a distant radio source.
v) In a typical two-way communication system (such as a mobile telephone system) signals are received (by a handset) from a point radio source (such as a base station) and transmitted back to that source. Embodiments of the present invention may be used to find the direction of the source using step iii) above and may then form an optimal beam in that direction using step ii). An antenna capable of performing this type of operation is known as a xe2x80x98smartxe2x80x99 or xe2x80x98intelligentxe2x80x99 antenna. The advantages of the maximum antenna gain offered by smart antennas is that the signal to noise ratio is improved, communications quality is improved, less transmitter power may be used (which can, for example, help to reduce irradiation of any nearby human body) and battery life is conserved.
vi) The addition of an internal or external monopole antenna can be used to null out the backlobe of the antenna, thereby reducing the irradiation of a person near the device, or to resolve front/back ambiguities in radio direction finding.